The present invention relates to devices or systems for monitoring the condition of a working fluid in a system, as for example, lubricating oil in an engine or power transmission device where the oil is subject to contamination by introduction of foreign substances such as engine coolant or deterioration due to chemical activity from products of combustion or aging.
In the operation of motor vehicles, it has long been desired to have the capability to monitor the fluid in the power transmission components such as the engine and power transmissions on a real time or running basis during operation of the vehicle and to have the capability to provide a warning or indication to the vehicle operator that the lubricating fluid has reached a state of contamination or deterioration so as to be considered unsuitable for continued operation. Heretofore, attempts have been made to utilize alternating current impedance spectroscopy to detect the change in impedance of a low level current flowing through a capacitor immersed in the fluid. Examples of such known systems employing AC electro-impedance spectroscopy are those described in U.S. Pat. No. 4,733,556 issued to Meitzler, et al., U.S. Pat. No. 5,274,335 issued to Wang, et al., and U.S. Pat. No. 4,646,070 issued to Yasuhara, et al. These systems describe the application of electro-impedance spectroscopy techniques for determining the condition of internal combustion engine lubricating oil due to contamination with engine coolant and to deterioration due to the chemical effect of the products of combustion and aging on the engine lubricating oil. The aforesaid known systems employ the technique of applying an AC signal current relatively high frequency to the plates of a capacitor and determining the change in the impedance measured thereon. Typically such frequencies are in the multi-hertzian range of generally between 10 hertz and 500 kilohertz.
One known report of earlier work is that described in the publication "The Applications Of AC Impedance Technique For Detecting Glycol Contamination In Engine Oil" by S. S. Wang, et al. published Jan. 4, 1997 by Elselvier Science S.A. This later work describes scanning the immersed capacitor with a sinusoidal voltage of 2.5 volts peak over a range of frequencies from one milli-hertz to one kilohertz. However this later work utilized only the bulk resistance of the fluid as determined by utilizing frequencies in the range between 100 hertz and 1 kilohertz and is not suitable for on-board continuous fluid monitoring.
However, none of the aforesaid prior art systems have provided a low cost and useable technique for on-board vehicle monitoring of the condition of the lubricating fluids and, thus it has been desired to provide a way or means of providing a simple low cost system for indicating contamination or deterioration of lubricating fluids, particularly those in motor vehicle engine and power transmission applications.